1. Field of the Invention
The present invention relates to an LC filter having printed coils, and more particularly to an LC filter which can be reduced in size without deteriorating the filter characteristics.
2. Description of the Related Art.
The LC filter is used mainly in high frequency circuits, such as FM receivers and televisions. Conventionally, the printed coils of such an LC filter are formed on an insulating substrate of alumina, mica or epoxy resin, or a magnetic substrate of a ferrite material.
The LC filter having the printed coils formed on the ferrite substrate can achieve a high inductance due to a high permeability of the ferrite material, but since the dielectric constant of the ferrite substrate is low, a capacitor must be formed of discrete parts, such as a plate type capacitor or chip parts, whereby the entire construction of the LC filter becomes larger. Also, when the insulating substrate is formed of alumina, mica or epoxy resin the capacitor must be formed of discrete parts.
On the other hand, in the filter using a dielectric of a high dielectric constant as the substrate, a capacitor having conductive patterns on both sides of the substrate can be formed, thereby effecting miniaturization of the LC filter, but the inductance of each printed coil becomes smaller due to the low permeability of the substrate.
In this case, to enlarge the inductance, the number of turns of the printed coils must be increased. Therefore, a dielectric substrate having a large surface area is required, whereby the entire construction of the LC filter device is enlarged. Furthermore, when the number of turns of the printed coils increases, a problem is created in that the filter characteristics are deteriorated due to stray capacitances formed by the dielectric constant of the dielectric substrate.